MBL 111B loudspeaker Measurements part 2

In the vertical plane (fig.6), the balance doesn't change significantly as long as the listener's ears are between the bottom of the larger Radialstrahler unit and the top of the smaller one. A suckout at the upper crossover frequency does appear 15 degrees below the optimal axis, but this is an impracticably low listening position for anyone other than a hobbit.

Fig.7 shows the spatially averaged response of the MBL 111Bs in my listening room. This kind of graph always reveals a nice correlation with a speaker's perceived balance, and fig.7 is no exception. The midrange is astonishingly flat, the bass is extended without being exaggerated in level, the low treble is a little recessed, and the penultimate octave peaks slightly—all exactly as I heard in my auditioning!

With its grille on, the 111B's impulse response (not shown) is obscured by high-frequency reflections from the perforated metal. Even with the integrating effect of the step response (fig.8), the strongest grille reflection can be seen as a discontinuity about 0.8ms after the tweeter step. Note also that, despite the physical setback of the higher-frequency drive-units, the use of a high-order crossover means that the 111B is not time-coherent (not that that really matters).

Fig.9 shows the MBL 111B's cumulative spectral-decay or waterfall plot with the grille in place. Note how ragged and hashy the sound decay appears in the top two audio octaves. This is due to the reflections of the sound from the grille. While I'm sure these occur too soon after the main arrival of the sound to be perceived as individual events, I'm also sure that the time-smearing of the treble energy results in the somewhat disappointing sound I heard from the speaker until I removed the grille. Fig.10 is a similar graph taken without the grille. You can see how much cleaner the entire treble region appears, though there is still a residual mode apparent just below 7kHz.

Finally, fig.11 shows the alteration to the MBL's on-axis frequency response. Obviously, the grille is the source of all the peaks and dips seen in the treble in fig.4. The speaker looks drop-dead gorgeous with the grille in place, but you need to remove it to get the sound quality you've paid for.—John Atkinson